This invention relates to an optical read-write apparatus, more particularly to an optical read-write apparatus for focusing two or more light beams of differing wavelengths onto one or more tracks on an optical medium such as an optical disk.
Optical read-write apparatus employing two light beams is known from the prior art. Apparatus has been described in which one beam is used for reading and the other for writing, or in which one beam is used for reading and writing and another beam with a different wavelength is used for erasing. The beams are focused by the same objective lens onto the same track, one ahead of the other, or onto a pair of adjacent tracks. In some systems it may be advantageous to have three or more beams, so that operations can be performed on three or more tracks simultaneously.
In such multiple-beam read-write apparatus it is necessary for each beam to follow its respective track accurately. Rough tracking control is generally performed by moving a carriage on which the objective lens is mounted to a desired position; then fine tracking control is performed by further movement of the carriage, by moving the objective lens from side to side, or by tilting a mirror. In this way it is possible to keep one beam accurately on track. The problem is how to adjust the tracking of the other beam (or beams, in apparatus with more than two beams).
Use of an in-line servo comprising a wedge prism rotatably mounted near the source of the second beam has been proposed. This system, however, tends to produce fatal additional tracking errors when the second beam is moved from track to track by in-line servo control of the wedge prism. In addition, the wedge prism and its driving mechanism are expensive and take up considerable space in the apparatus.